1. Field of the Invention
The present invention is related to a process for making an elastic polyurethane fibers, and the elastic polyurethane fibers prepared by the above method. More specifically, the present invention relates to a process for preparing an elastic polyurethane fiber, including mixing a polystyrene polymer or copolymer having a number average molecular weight of 50,000–500,000 with polyurethane having a number average molecular weight of 15,000–100,000, followed by a spinning process; and thusly preparing elastic polyurethane fiber.
2. Description of the Related Art
Generally, polyurethane is obtained by firstly polymerizing polyol as a diol compound having a high molecular weight with excessive diisocyanate to form a prepolymer of an isocyanate-terminated polyol, which is then dissolved in a proper solvent, and added with a diamine- or diol-based chain extender for a second polymerization.
For the preparation of a polyurethane fiber, there are a dry spinning process, a wet spinning process, a chemical spinning process, or a melt spinning process. In the dry spinning process, a polyurethane is prepared in the melt, dissolved in a solvent to obtain a polyurethane solution, and then dry spun. Further, in the wet spinning process, a polyurethane is synthesized directly in a solvent. As it is formed, the polyurethane dissolves in the solvent to form a solution that is then dry spun. Also, the chemical spinning process is carried out by extruding an isocyanate-terminated prepolymer solution into an amine-based chain extender solution through a nozzle to effect a chain-extension.
In addition, the melt spinning process is performed by extruding melted polyurethane through a nozzle, and cooling the extruded polyurethane.
Thusly prepared polyurethane fiber may be mixed with other fibers, for example, polyacrylonitril fiber, wool, cotton, silk, etc., according to desirable end uses. As used in common, spandex has its meaning, that is a manufactured fiber in which the fiber-forming substance is a long chain synthetic elastomer comprised of at least 85% by weight of a segmented polyurethane.
The polyurethane fiber is variously applied, due to unique properties, such as high elasticity. According to the enlargement of applicable fields, there are required polyurethane fibers further having novel properties in addition to conventional properties. By this time, polyurethane fibers have been developed to have further enhanced heat resistance and elasticity and, in recent years, demands for polyurethane fibers having excellent heat set properties become gradually increasing.
Therefore, intensive research into improvement of heat set properties of the polyurethane fiber has been conducted. In this regard, Japanese Patent Publication Nos. Sho. 63-53287 and 63-53288 disclose method of improving heat set properties of an elastic polyurethane fiber by use of specific additives, such as a diisocyanate dimer or powdered silica. However, the above method is disadvantageous in that the additives are inconsistently dispersed into the polyurethane solution. Further, it is difficult to choose a proper mixing condition, and the polymerization process by itself becomes complicated.
In addition, Japanese Patent Publication No. Sho. 43-639 discloses a method of preparing polyurethane, including reacting a mixture of a polymer diol and a low molecular weight diol 1–3 times as much as the mol of the polymer diol, with a diisocyanate compound, to prepare a hydroxide-terminated prepolymer and an isocyanate-terminated prepolymer, which are then reacted together to produce a polyurethane polymer. However, thusly obtained polyurethane is subjected to only a melt spinning process.
Further, Korean Patent Laid-Open Publication No. 2001-5854 discloses a method of preparing an elastic polyurethane fiber, characterizes in that two prepolymers, each having an isocyanate termination and a hydroxide termination with relatively similar viscosities are reacted together to obtain a liquid polyurethane polymer, which is then continuously extruded through a nozzle. According to this method, since the two prepolymers have similar viscosities, it is possible to uniformly mix each other upon a second polymerization, thus increasing the mixing effect and spinning stability. However, the above method is more complicated process, compared to conventional processes of preparing an elastic polyurethane fibers. As well, since the mixing effect and spinning stability are obtained by controlling the conditions of the process, the above method is not practical.
Moreover, Japanese Patent Laid-open Publication No. Hei. 7-316922 discloses a relatively simple method capable of improving heat set properties of an elastic polyurethane fiber, in which when the polyurethane fiber is prepared by a conventional process, a part of a polyurethane component is replaced with a thermoplastic polyurethane resulting from the use of a diol as a chain extender of the second polymerization. The elastic polyurethane fiber prepared by the above method has an improved heat set properties, but is decreased in elongation, with a drastic increase of modulus.
Also, there is described a method of replacing a part of a conventionally prepared polyurethane by a low molecular weight styrene maleic anhydride-based copolymer having a number average molecular weight less than 5,000. However, with the intention of improving heat set properties and maintaining fundamental properties of the fiber, the above method is not preferable.
In Korean Patent Laid-open No. 2001-16788, there is disclosed a method of improving heat set properties of a polyurethane fiber by following an additional thermal treatment after a dry spinning. However, this method is disadvantageous in that other devices are further required, in addition to a conventional spinning device. Additionally, the fiber is excessively heated, thereby improving the heat set properties but decreasing fundamental properties, such as elongation and modulus.